An example of an optical memory which avoids the problems with mechanically based addressing methods is disclosed in International Published Application WO93/13529. Data are stored in an optical layer 19 which is capable of selectively altering light by e.g. a change in transmissivity, reflectivity, polarization and/or phase. The data layer 19 is illuminated by controllable light sources 15 and an array 21 of imaging lenslets project the image onto a common array 27 of light sensors. By selectively and sequentially illuminating different data regions or pages in the data layer, correspondingly different data patterns are imaged by corresponding lenslets 21 onto the common sensor array 27.thereby enabling the retrieval of a great number of data pages by multiplexing electrooptically. In a preferred embodiment the lenslets 21 may be replaced by diffractive optical structures 402, 406, although it is acknowledged that unless monochromatic or narrow band light sources are used, diffractive optical structures give rise to undesirable aberrations or distortions in the image data due to different source wave lengths. Moreover, this optical memory also structurally separates the read and write optics, leading to a rather complicated optical arrangement involving the provision of a beam splitter 31 in the housing 11 of the memory.
As an example of a data storage medium suitable for an optical memory reference may be made to e.g. U.S. Pat. No. 5,436,871 (Russell), which derives from the parent application of WO93/13529 and discloses a compact optical memory wherein data is stored on a card 104 with integrated lenslet arrays 210 and in an optical data 190 layer capable of selectively altering light by a change in transmissivity, reflectivity, polarization and/or phase. Optical memories may, however, also be implemented with a storage medium capable of emitting fluorescent light upon excitation with a suitable light source, as for instance disclosed in the above-mentioned International Published Application WO96/37888, or containing a chromophore compound, as for instance disclosed in International Published Application WO96/21228 which teaches the use of bacteriorhodopsin as the chromophore.